Bug 1547416 - Move nsIPresShell::ScrollAxis to PresShellForwards.h r=smaug
`nsIPresShell::ScrollAxis` can be used anywhere and it's used by some
utils actually. So, it should be in `mozilla` namespace and perhaps,
`PresShellForwards.h` is a good place to move it rather than creating
new header file.
Differential Revision: https://phabricator.services.mozilla.com/D29110